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HomeTips & GuidesTroubleshooting Trailer Wiring Harness: A Comprehensive Guide

Troubleshooting Trailer Wiring Harness: A Comprehensive Guide

About Troubleshooting Trailer Wiring Harness, Trailers are lifesavers when carrying heavy loads like tools for farming, construction, landscaping, etc. Hence, you must always strive to maintain the trailer and keep it in good shape.

One of the things to frequently examine is the brake lights. At times, the turn signals may grow dim, or your lights stop working/work ineffectively as expected. 

Undoubtedly, such circumstances are frustrating. Luckily, we have the solution you need. Today’s post will concisely take you through troubleshooting the trailer wiring harnesses.

Table of Contents

The Importance of Trailer Wiring

Well-functioning trailer lights ensure the safety of you and other road users. If your lights do not work efficiently, it might culminate in a catastrophic situation for you, the pedestrians, and other drivers.

Therefore, constantly check the trailer light wiring to ensure the uttermost attentiveness and cautiousness while on the road. 

Truck trailer triple rear lights

 (Truck trailer triple rear lights)

How to Fix Common Trailer Wiring Problems

First and foremost, separately test your trailer and vehicle to annihilate common problems. In that way, you’ll determine if the problem lies with the vehicles’ trailers depending on their wiring systems. 

Next, follow the step-by-step instructions to successfully fix your trailer wiring issues. 

Testing for Proper Functions

The first step is removing any power fuse for some minutes before reinstalling it. Then, confirm how your vehicle’s lights function using a 12V circuit tester in a 4-way plug. 

If you get the correct light wiring functions, check the trailer’s wiring system. However, if there’s a malfunction, examine the wiring going into the vehicle’s converter box. 

Generally, signals from the car or truck should go to the converter box, so ensure this is the case. Behind the box in a two-wire vehicle, a yellow and green wire shows brake lights and turn signals.

On the contrary, a three-wire car has green and yellow wiring for turn signals and red wires for brake lights.

If you notice improper power on the light’s functions, consider the following;

  • Wires are incorrectly attached to the trailer.
  • Loose or weak ground connected on the hand-wired harness.
  • Missing relays/fuses or loose wires/connectors on the plug-in harness. 

Confirming Accurate Wire Connections

Secondly, note that each wire in a hard-wires installation connects in the rightful place according to the system in use. For instance;

  • The 5th wire in a trailer with a 5-way harness wire joins the reverse light circuit.
  • Then, your vehicle’s positive battery terminal can connect a 12V power wire.
  • Also, the brake wire will have a white color grounding should the vehicle have both turn signals and brake lights.

For a plug-in installation, do the following;

  • Examine broken locking tabs, bent or loose pins, and damaged or loose wires/ connectors. 
  • Remove and connect the connectors again to ascertain that they click together.
  • The harness connectors should be on the required side of the vehicle, i.e., yellow wires on the driver’s side and green on the passenger’s. 
Wiring harnesses

(Wiring harnesses) 

  • Ensure the part number corresponds to your truck’s correct year, model, and make.
  • If you operate via a factory tow package harness, establish that your vehicle has the package.
  • Fasten the connection between the vehicle battery and the 12V power wire when looking for any blown a fuse. 

Additionally, you can do a continuity test to check for broken wires. 

Inspecting Ground Connections

Thirdly, examine the ground region on your vehicle to see whether there’s paint buildup or corrosion. If there’s an issue, replace the corroded ground screws or clean the area until you reach the surface.

Be careful not to stack additional ring terminals below the ground if there’s a factory ground screw. However, should there be a stacking, make way for the harness by moving the ground towards the bottom or in another location. 

In addition, the ground wire should always connect to the trailer frame. Ideally, the connection should be at the back of the tongue fold if your trailer has one. If the ground wire joins an aluminum area, you can shift it to the frame. 

Looking for Overload Conditions

An overload can cause melting due to overheating. Therefore, know beforehand the maximum amperage rating of the harness while comparing it to the one on the trailer lights. 

Furthermore, you can remove the fuse, leave it for some time, then repower it to reset the power unit. Remember the circuit test for the 4-way plug function before plugging in your trailer. 

Other Possible Trailer Light Wiring Issues

It includes;

A buildup of elements causes white or green corrosion in some parts of the trailer. You can clean the trailer using a battery terminal cleaner or spray the connector with an electrical contact cleaner.

After the spray, clean all the contact pins using a fine wire brush. Alternatively, replace the plugs with newer ones or use 220-grit sandpaper to scrap off the corroded parts. 

A trailer with corrosions

 (A trailer with corrosions)

How to Test trailer lights with a multimeter

Troubleshooting Trailer Wiring Harness: Briefly

  • Disconnect the wiring system in the trailer.
  • Next, place the multimeter probes on the ground. The ground connection will help you know if you have a problem with the trailer plugs or connectors.

Troubleshooting Trailer Wiring Harness: For connectors’ testing

Set the multimeter to Ohms, then place the black probe on the negative pin. Ground the positive probe connection on a metal area and test for any problems. Expect a reading of 0.3 ohms. 

Later, position the red probe in the positive pinholes and the black probe on the negative pin. Expect a reading of 3-Ohms. 

Troubleshooting Trailer Wiring Harness: For trailer plugs

Place the red probe on the positive pins and the black probe on the negative hole to add the connection of different lights. A voltage reading of 12 should be okay. 

A digital multimeter

 (A digital multimeter)

Frequently Asked Questions

Often, such a phenomenon occurs due to two things. Firstly, your system could have weak ground. Secondly, it could have a not-yet-noticed short circuit.

As such, we advise you to countercheck and test the vehicle’s wiring and connector for maximum functionality. 

Q: All the lights on the trailer are out. What causes it?

The main problem here is the ground system which has enough power to offer some lighting functions though not all. 

To check for weak grounds, you can look at some of the places or conditions below;

  • A wire connected to a surface/trailer body with rust, paint, or undercoating. Such elements interfere with the ground.
  • Design of the trailer. A tilting bed would mean the ground isn’t getting through the pivot point.

Q: The turn signal dims and flashes rapidly when I connect my trailer to the vehicle. What causes it?

Generally, the turn signal ampere load often doubles when you connect trailer lights to the system in your vehicle. Then, manufacturers usually place flashers in vehicles that help regulate blinks of turn signals. 

However, an additional load of trailer lights may overburden the standard flasher. Thus, we recommend ensuring the vehicle engine is running and replacing the basic flasher with a heavy-duty flasher (approximately 8-10amp).

Ultimately, a working engine increases the lamps’ brightness while the flasher delays your turn signals. 

Q: Turn signals don’t work when headlights are on

Again, the issue might be a weak ground. Refer to question two for further explanation and remedy. 

A flashing turn signal

(A flashing turn signal)


Now that we’ve given you tips and tricks on troubleshooting your trailer wiring harness, we hope you’ll peacefully enjoy your ride. However, please get in touch with us if you still have questions on today’s topic. We’ll be more than happy to help. 

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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